6
dc2181afb
DEMO MANUAL
DC2181A-A/B
theory of operation
The waveforms in Figure 8 were captured at a transmit-
to-receive gap of 8mm. The blue trace is the waveform at
the C
X
pin of the receiver board (see Schematic: 400mA
Wireless Synchronous Buck Battery Charger), and the
red trace is the charge current into the battery. Although
the transmit waveform is a sine wave, the series-parallel
connection of the secondary resonant circuit does not
yield a sine wave, and this waveform is correct. The charge
current into the battery has an average of ≈ 400mA, for a
delivered power of 1.5W (V
BAT
= 3.7V). However, 10mA
has been diverted to the charge LEDs, for a net battery
charge current of 390mA. The ripple on the charge current
is synchronous to the transmit waveform.
Summary
The LTC4120 wireless power receiver IC adjusts the
receiver resonant frequency to keep the system from
transferring too much power when the coupling is high
between transmit and receive coils. The LTC4120 wireless
power receiver IC increases power transfer when power
transfer is insufficient. This is accomplished by switching
capacitors into the resonant circuit using the DHC pin. This
gives a much wider operating transmit distance.
Figure 9 shows V
IN
to the LTC4120 and the battery charge
current. The blue trace is the charge current into the bat-
tery, and the red trace is the voltage at V
IN
on the receiver
board. V
IN
is about 25V, while the LTC4120 delivers 1.5W
at a distance of 8mm, to the battery. There is negligible
transmit frequency ripple on V
IN
, and the voltage is well
above the 14V DHC voltage. This indicates that the input
rectifiers are operating in peak detect mode, and that DHC
is inactive.
Figure 9. DC2181A Receiver
2µs/DIV
V
IN
TO GND
5V/DIV
DC2181A F09
I
BAT
100mA/DIV
V
BAT
= 3.7V
Figure 8. DC2181A Receiver Board
2µs/DIV
I
BAT
V
BAT
= 3.7V
100µA/DIV
DC2181A F08
Cx TO GND
20V/DIV
DHC
When V
IN
is above 14V, the DHC pin is open and C2P
doesn’t enhance the energy transfer; this is the detuned
state, and the resonant frequency of the receive tank is
142kHz. When V
IN
falls below 14V, the DHC pin is grounded
putting C2P in parallel with both C2S and AE1 thus chang-
ing the resonant frequency to 127.4kHz. When the receiver
is tuned at 127.4kHz and drawing significant power, the
transmit frequency is pulled down to 127kHz. So, at full
power the system is now a double-tuned resonant circuit.
Figure 10 shows approximate power transfer vs distance
between transmitter and receiver. Note the minimum
clearance. The minimum is needed to avoid exceeding
the maximum input voltage.
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